1. Field of the Invention
The present invention relates to the field of structural reaction injection moldings. More particularly, the subject invention pertains to high density fiber reinforced polyurethane/polyisocyanate parts especially adapted for automotive applications requiring a "near Class A" or a "Class-A" surface appearance. Such high density fiber-reinforced reaction injection molded systems are known as high density structural-RIM or high density SRIM.
2. Description of the Related Art
The growth of high density SRIM in the automotive industry has been limited by the inability to achieve "Class A" surface finishes in exterior applications. For the most part, high density SRIM parts have been relegated to use in hidden part applications such as bumper beams, floor pans, and chassis components. However, SRIM offers many advantages over conventional molding processes, such as high manufacturing volumes due to short cycle times, low labor costs, relatively low molding pressures and low mold temperatures in the manufacture of high strength composite materials.
The sheet molding compound (SMC) process has been used extensively as a compression molding process to manufacture exterior body parts. In the SMC process, a sheet of material typically made of calcium carbonate filler, chopped strand glass, unsaturated polyesters, and various additives dissolved in styrene is charged into a mold heated at temperatures from 284.degree. to 392.degree. F. (140.degree. to 200.degree. C.), and the mold is closed under a press tonnage of about 2000 tons. Molding pressures reach from 1000 to 4000 p.s.i. The high mold temperatures activate a reaction between the styrene and the unsaturated polyester to crosslink the final material, providing a "near Class A" or "Class A" surface. As a result of the large quantity of glass and calcium carbonate in SMC (typically around 76 wt. %), overall thermal shrinkage arising from the difference in the coefficient of linear thermal expansion (CLTE) between the polymer matrix and the glass/fillers is small. However, in the SMC process, large quantities of glass/filler are needed to achieve a "Class A" finish, which raises costs, and does not have the advantage of higher strengths achieved by the presence of a reinforcing mat as in the SRIM and RTM processes. Furthermore, tooling and energy costs are high due to the large molding pressures, press tonnage, and high mold temperatures required.
An alternative process gaining considerable growth as a method of manufacturing exterior body panel parts is the Resin Transfer Molding (RTM) process. The materials of manufacture in RTM include polyesters, vinylesters, or epoxy resins. In the reinforced RTM process, low viscosity reactants are statically mixed and injected at approximately 50 to 200 p.s.i. into an optionally heated mold containing a reinforcing mat. Curing times range from 5 to as long as 30 minutes. As a result of the low molding pressures and the generally unheated mold, tooling and energy costs are low. The slow cure times permit one use a slow rate of injection, thereby ensuring that air entrainment is kept minimal. The drawback to this process, however, is that long cure times considerably reduce manufacturing volumes.
The SRIM process is advantageous over the SCM process in that high manufacturing volumes can be achieved both at low molding pressures ranging from 200-220 p.s.i., thereby reducing tooling and equipment costs, and at low molding temperatures ranging from 140.degree.-180.degree. F. (60.degree.-82.degree. C.), thereby reducing energy costs. SRIM parts contain a reinforcing mat which can greatly improve the tensile strength and flex modulus of the final part. Unlike the RTM process, the matrix polymer in SRIM is typically polyurethane/isocyanurate, which has a very short cure time ranging from 45 seconds to 2 minutes, advantageously resulting in high manufacturing volumes. In high density SRIM, the polyisocyanate "A" side and the polyol resin "B" side are impingement mixed and injected into a heated mold containing a reinforcing mat. Since such compositions cure so quickly, high injection rates are required, which unfortunately cause air bubbles to become trapped in the molded part. Upon baking the finishing paint onto the molded part, these air bubbles rise to surface and/or cause the polymer matrix to separate from the reinforcing mat, reducing strength and causing blisters to appear on the surface of the final part. A blistered surface on a baked part is one factor hindering a from attaining a "Class A" finish. Reducing the injection rate to prevent air entrainment, however, would negate the high manufacturing volumes associated with the SRIM process.
Besides the problem of air entrainment encountered in the high density SRIM process, another disadvantage faced by both SRIM and RTM processes is that glass reinforcing mats often abrade and scratch mold surfaces, which in turn are transferred to the molded article and ruin an otherwise "Class A" finish. To solve this latter problem, one may employ a chopped fiber reinforcement process (RRIM) where the short glass and/or mineral fibers are directly incorporated into the polyol resin "B side" of the two component system rather than laid on a mold surface. However, the chopped fibers not only raise the viscosity of the system thereby seriously affecting the ease of processing, but also reduce the strength of the molded article as compared to mat reinforced RTM and high density SRIM. Parts produced by RRIM are largely unusable for automobile hood, roof top, and trunk deck applications due to their low mechanical properties, and are instead relegated to semi structural applications.
Overall, the SRIM process offers low tooling, equipment, and energy costs; high manufacturing volumes; and produce parts with high tensile strength and flexural modulus. A major drawback preventing these parts from being used as hood, trunkdeck, and exterior body panels is that the surfaces of these molded parts blister upon exposure to the high bake temperatures in paint ovens.